If you attempt to transfer account ownership to another user, and you receive an error that reads, "cannot transfer account ownership to selected user as he is part of multiple organizations," it means that the person you are attempting to transfer ownership to is already a user of someone else's account.
You can transfer ownership only to someone who is not already a user for any other account owner. You must first have the account owner delete the user from their account. Once the user is removed from all other account owner's accounts, you can transfer ownership successfully.
For more information on this topic, see Switch account ownership to another user.